This invention relates to a method for preparing attrition resistant fluid bed catalysts useful in the production of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly it is directed to the preparation of fluid bed catalysts suitable for producing maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or a mixture thereof.
The advantages of fluid bed hydrocarbon oxidation processes compared to fixed bed hydrocarbon oxidation processes are well known in the art, including the improvement of temperature control and heat transfer for oxidation reactions. Catalysts which are suitable for fixed bed processes, however, are not necessarily suitable for fluid bed processes. Catalysts which are suitable for fixed bed processes in which there are few attritting forces in operation may be too "soft" to withstand the attrition caused by fluid bed operation.
Attrition of the catalyst in fluid bed operations, that is, the removal of the outer layer of the catalyst particle by abrasion or the breakup or fracture of the catalyst particle is caused by impact of the fluidized particles with each other, with the walls of the fluid bed vessel, especially within the reactor cyclones which trap the fluidized catalyst particles before they escape the reactor to return the particles to the catalyst bed.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve reducing a pentavalent vanadium compound, and combining the same with a phosphorus compound, and if desired, promoter element compounds under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered and calcined, before or after the fixed bed catalyst particles are formed, to provide active catalytic material.
U.S. Pat. Nos. 3,888,886; 3,905,914; 3,931,046; 3,932,305 and 3,975,300 disclose the testing of promoted vanadium phosphorus oxide catalysts for maleic anhydride production from butane in one inch diameter fluid bed reactors. In most instances, the catalysts were prepared by forming the catalyst precursor in aqueous media (in U.S. Pat. No. 3,975,300 the precursor was formed in a paste of a vanadium compound, a phosphorus compound and an organic reducing agent), drying and thereafter grinding and sieving the precursor to a powder of about 74 to 250 microns size. This manner of preparation, however, does not obtain the attrition resistant catalyst particles preferred for successful fluid bed operation.
Commercial fluid bed catalysts are preferably microspheroidal particles within the range of about 20 to about 300 microns in average diameter, preferably having about 80% of the particles within the range of about 30 to about 80 microns in diameter. Most preferably, about 25 to about 40% of the particles have an average diameter of less than 45 microns.
It is therefore an object of the invention to provide a process of preparing attrition resistant fluid bed vanadium and phosphorus mixed oxide containing oxidation catalysts.
It is a further object of the invention to provide a process for producing maleic anhydride from 4-carbon atom hydrocarbons utilizing attrition resistant fluid bed vanadium phosphorus mixed oxide catalysts.